The present invention relates generally to mixer circuits and particularly to a direct conversion mixer for use in mobile communications.
A direct conversion system is known which converts a signal modulated into a radio frequency directly to a baseband signal and modulates a baseband directly to a radio frequency. Generally, this system does not provide an intermediate frequency. As such, it can dispense with image suppression, reduce spurious source, an d simplify a system advantageously. It is also known, however, that the system provides 2nd order intermodulation distortion, local oscillation wave noise, and other similar disadvantages. One approach to alleviate these disadvantages is an even harmonics mixer such as an anti-parallel diode.
In recent years there is a demand for miniature and inexpensive ICs for high-frequency radio communications, as represented for example by mobile phones. In such a market, current consumption reduction, IC yields and the like are important issues.
In an even harmonics mixer, a mixer circuit itself does not consume a current. To operate the mixer circuit, however, a high local oscillation wave input level is required. Accordingly, overall current consumption reduction has been an issue to be addressed. Furthermore there is also a demand for reducing an influence of semiconductor process variation.
As an approach to overcome such disadvantages, a system using a transistor""s diode characteristics has been considered in recent years.
FIG. 5 is a circuit diagram showing one example of a conventional even harmonics mixer using transistors. As shown in the figure, transistors 1 and 2 have their respective collectors connected together and their respective emitters connected together to configure a local frequency multiplication unit. A resistor R1 is connected between the collectors and a power supply and a resistor R2 is connected between the emitters and ground. Transistors 1 and 2 receive a modulated wave signal BB at their respective emitters and receive signals LO and ↓LO (xe2x80x9c↓xe2x80x9d indicates an inverted signal) each having a component of a frequency of a local oscillation wave inverted in phase at their respective bases. Thus the local frequency multiplication unit generates a local oscillation wave frequency component multiplied by two and can extract a desired frequency component by multiplying an input signal frequency and the local oscillation wave frequency component.
The mixer of FIG. 5, however, is associated with a small power gain and requires a high local oscillation wave input to operate the local frequency multiplication unit.
A main object of the present invention is therefore to provide in the field of high-frequency radio communications a mixer circuit suitable for applications for which there is a demand for low frequency currents, high IC yields and simplified system configurations.
In accordance with the present invention a mixer circuit includes: a local frequency multiplication unit configured of a first transistor having an input electrode receiving a frequency component of a local oscillation wave signal and a second transistor receiving a signal opposite in phase to the local oscillation wave, the first and second transistors having their respective first electrodes connected together and their respective second electrodes connected together, the local frequency multiplication unit outputting a modulation signal at the first electrode of each transistor; a third transistor connected in a pair with each of the first and second transistors of the local frequency multiplication unit, having an input electrode receiving a reference signal, and a first electrode differentially outputting the modulation signal; a voltage source providing the first, second and third transistors at their respective first electrodes with a predetermined voltage; a signal input unit connected to the first, second and third transistors at their respective second electrodes and receiving a modulated signal; and a constant current source connected to the signal input unit.
Furthermore, the signal input unit is a fourth transistor having an input electrode receiving the modulated signal, a first electrode connected to the second electrode of each of the first, second and third transistors, and a second electrode connected to the constant current source.
Furthermore, there are provided two sets of the local frequency multiplication unit, the third transistor for reference and the fourth transistor, the voltage source is connected to the two local frequency multiplication units commonly, and the constant current source supplies a constant current to the two fourth transistors commonly.
Furthermore, the constant current source is a variable constant current source.
Furthermore, the mixer circuit further includes a variable current source varying a bias current provided to the input electrode of the third transistor.